1. Field of the Invention
The invention relates to a hydrodynamic coupling, that is a turbo-engine, that works according to the Föttinger principle in order to transmit drive power hydrodynamically and thus wear-free
2. Description of the Related Art
Hydrodynamic couplings have been known for decades. They have an impeller and a turbine wheel that form a toroidal working chamber that is filled with or can be filled with working medium. One skilled in the art distinguishes between constant filling couplings and flow-through couplings. In constant filling couplings, the working medium is always disposed within the hydrodynamic coupling. In flow-through hydrodynamic couplings, an external working medium cycle is connected to the coupling so that the working medium flows into the hydrodynamic coupling via an inlet and is conducted out of the hydrodynamic coupling via an outlet.
Hydrodynamic couplings are used in motor vehicles, especially in the drive train, for instance for starting up the vehicle, and in stationary systems for wear-free transmission of drive power to a driven engine. The former couplings can be called vehicle couplings, and the latter can be called industrial couplings.
When hydrodynamic couplings are employed in the drive train of a motor vehicle, for instance as starting couplings, it is fundamentally desirable that the coupling is able to transmit a high power or a high torque from the driving side (the impeller) to the output side (the turbine wheel) using the hydrodynamic working medium cycle, as a rule an oil or water cycle. Thus a hydrodynamic coupling is advantageously designed such that it has the highest possible λ progression or torque progression over the speed ratio v or the slip s. Torque progression is the progression of torque that is transmittable at a specific speed ratio or a specific slip using the hydrodynamic coupling. The performance figure λ is a characteristic parameter of the hydrodynamic coupling that is known to one skilled in the art. The definition of λ is provided for instance in Dubbel, Taschenbuch für Maschinenbau [Pocket Book of Engineering], 18th Edition, page R 50. It is calculated from the torque applied to the impeller or from the power applied to the impeller relative to the density p of the working medium, the cycle diameter D, and the angular velocity co of the pump:λ=[(Mp)/(ρD5ωp2)]=[(Pp)/(ρD5ωp3)]
The speed ratio v is calculated from the speed of the turbine wheel divided by the speed of the impeller. The slip s in percent is calculated as follows:s=(1−v)×100
However, a high torque progression or λ progression for a hydrodynamic coupling can lead to the hydrodynamic coupling causing the engine to stall when there is high slip, for instance at 100 percent slip, during acceleration of the vehicle from zero or a slow engine speed because the torque transmitted by the hydrodynamic coupling and against which the engine is working is too great. In order to prevent this, in the past the λ progression has been reduced adequately by a corresponding reduction in the size of the hydrodynamic coupling or a reduction in the capacity of the hydrodynamic coupling so that there is no question of the engine stalling, even at 100 percent slip.
In FIG. 1, a solid line depicts a λ progression or torque progression (M) over slip s or speed ratio v that prevents the drive engine from stalling. A dashed line is used to represent a λ progression or torque progression that improves the coupling properties in a low slip range or great speed ratio compared to the progression represented by the solid line. However, this progression depicted with the dashed line could lead to the drive engine stalling in a high slip range.
A performance characteristic or λ progression for the hydrodynamic coupling that combines the advantages of both aforesaid design characteristics would be desirable. This is true both for hydrodynamic couplings in motor vehicles and in stationary systems (industrial couplings).
What is needed in the art is a hydrodynamic coupling that avoids the aforesaid problems and satisfies the requirements described.